The present invention relates to an improvement of the control device for an automatic transmission gear system and aims to alleviate the shock caused by gear shifting.
Automatic transmission gear systems used in automobiles are generally capable of selectively actuating a plurality of frictional elements such as clutches and brakes that are provided in the transmission gear system by oil pressure in order to obtain different gear ratios, and for effecting the gear shift, one of the frictional elements is disengaged while another one is engaged.
In an automatic transmission gear system of the above construction, adequate control of timing for disengaging one of the frictional elements and engaging another element is of great importance in alleviating shock due to speed changes and in assuring comfortable driving as well as preventing any damage to the transmission gear system.
For example, if the disengagement occurs too early and the engagement too late, damage occurs during the time between the disengagement and the engagement. On the other hand, if the disengagement occurs too late and the engagement too early, a state of both frictional elements being engaged occurs. This leads to contradictions in a speed change gear assembly and causes damage to the vehicle as a whole as well as putting an excessive load on the frictional elements and the gear systems, thus inducing damage thereto.
Therefore, when the gear ratio of the transmission is changed from lower to higher speed with the engine in a driving state, an overlap of torque capacities of the frictional elements proportional to the output torque of the engine is desirably provided between the disengagement and engagement to prevent the engine from racing; while on the other hand, when the engine is in the driven state, it is preferable to provide a suitable time interval after the frictional element for lower speed driving is released until the rotational speed of the engine decreases enough to accommodate the driving condition at a higher speed. In this way, the frictional elements for higher speed driving can be engaged (the interval is defined as a zero overlap or an overlap of negligible amount).
On the contrary, when the gear ratio of the transmission is changed from higher to lower speed with the engine in a driving state, it is desirable that the timing for disengaging and engaging the frictional elements be controlled corresponding to the driving speed of the vehicle. When the vehicle is running at a higher speed, it is preferable that, after disengaging the frictional element for higher speed driving, a small amount of said overlap be provided to let the rotational speed of the engine increase enough to accommodate the driving condition at a lower speed, before the frictional element for lower speed driving is engaged. As the vehicle slows down, it is necessary to increase the amount of the overlap. Especially at a low speed, it is desirable that the interval is reduced to almost zero, and the overlap increased.
Thus, it is necessary to provide the overlap at lower speeds and reducing it gradually as the vehicle gathers speed. However, if the overlap of the engagement is small when the engine is in a driven state, an interval takes place causing the engine braking effect of the vehicle to be cancelled for a while, and thereafter the frictional element for lower speed driving is engaged causing the vehicle to come under forcible engine braking effect, resulting in impaired comfort of driving.
A one-directional brake instead of ordinary brakes can be used to automatically change such gear ratios, but this cannot be adapted to all of the frictional elements in the transmission. Also, a control device activated by oil pressure can be used which detects the change in the rotational speed of axis of the transmission gear system so that the oil pressure can be supplied without delay to the frictional elements for coupling when synchronization is attained (Jap. Pat. Pub. Sho 54-35631).